When collecting current data in ENU coordinates it is crucial to have accurate values for the instrument orientation. In order for the instrument heading to be reported correctly a compass calibration is needed. Each compass has been calibrated at the factory to quantify the characteristic response of the individual components and of the system as a whole. When it leaves the factory, each system can measure its tilt and the direction of its magnetic field vector accurately, anywhere in the world. However, users disturb the magnetic field near the instrument when they deploy. Adding a battery pack and mounting the instrument with deployment hardware adds magnetic materials that change the magnetic field around the instrument. The compass calibration procedure quantifies this magnetic disturbance, which is then used to correct the obtained heading.
As a side note, the compass is not used when measuring velocity in XYZ or beam coordinates, but if you plan on using the compass heading at a later point (for instance to orient the XYZ velocities relative to the lake axes) it is probably worth calibrating the compass in advance.
Calibration setup
Compass calibration should be performed immediately before deployment, especially if new magnetic materials have been introduced near the instrument. This ensures that any local magnetic disturbances, particularly those rotating with the instrument, are accurately corrected. The calibration process involves a single, slow rotation of the instrument around its tilt axis, lasting at least 60 seconds. For the calibration to be effective, the compass and any nearby magnetic materials must remain fixed relative to each other throughout the procedure. When properly executed, this process can compensate for magnetic disturbances even greater than the Earth’s magnetic field.
It is essential that the compass calibration is performed using the same setup intended for deployment. This means that the battery must be inserted, and if a mooring frame is being used, the instrument should be mounted to it during calibration. This ensures that any magnetic influences introduced by the full deployment configuration are properly accounted for, resulting in a more accurate and reliable calibration. A steady rotation of the entire deployment setup can proof to be difficult for larger frames and instrument. Customers have been successful by mounting wheels on the mooring frame, suspending the setup from a tree or rotating the assembled buoy in water using a vessel from a considerable distance.
Avoiding magnetic interference
Be aware of any material that may be disrupting the magnetic field in the vicinity during calibration as this may skew the directionality of your data. This is likely to occur at magnetic fields stronger than 5 Gauss. You should conduct this procedure outdoors, away from other possible magnetic elements. Remove any devices from the vicinity that could emit a magnetic signal like phones and laptops not required for the calibration. If possible, do the final calibration before you board your vessel as ship decks tend to have non-uniform magnetic disturbances significant enough to disrupt the compass calibration process. Do not subject any part of the instrument, including batteries, to a magnetic field stronger than 20 Gauss. This may permanently damage the components and cause irreversible damage to the compass.
Identifying the instrument's z-axis
You can identify the instruments z-axis by first locating the arrow indicating beam 1 / the x-axis. Use the right-hand-rule to remember the notation conventions for vectors. Use the first (index) finger to point in the direction of positive X-axis and the second (middle) finger to point in the direction of positive Y. The positive Z-axis will then be in the direction that the thumb points. A heading of 0° should therefore be observed, if if the instruments x-axis is pointing towards North, consequently a heading of 90° indicates the x-axis pointing towards East. Use the pitch and roll output of the software to determine, weather the instrument's position is steady and horizontal.
Calibration procedure
Connect the instrument and select Compass Calibration in the Deployment Software suitable for your instrument. To find the correct software, follow the information on our software page. Within the calibration routine it is possible to record the calibration data to a .txt file. This can later be used to apply the same calibration to the same or another instrument.
The steps below apply both to instruments with a standard magnetometer and to those with the AHRS option.
Assemble the deployment setup.
Mount the instrument in the frame/buoy together with the battery canister, ballast, and any other components, so the setup matches the intended deployment as closely as possible.Place the system in a suitable calibration area.
Keep the system away from magnetic disturbances. Strong magnetic fields (above approximately 5 Gauss) can distort the calibration. Avoid large steel objects and electronic devices such as phones. Perform the final calibration before boarding your vessel, as ship decks often contain magnetic disturbances that can disrupt the calibration process. Best calibration results can be achieved when suspending the frame in air or in a rotatable structure. If that is not possible, place the instrument setup on a flat surface.Prepare to rotate the system and start the calibration
Make sure you can rotate the entire assembled system slowly around the instrument’s Z axis. Click Start.Rotate one full turn.
Rotate the system 360° around the instrument Z-axis at a steady pace. Note that when doing this in the field, you cannot expect to produce a perfect circle. However, we recommend you do this slowly, approximately 30 seconds per 360º, in an attempt to come as close to the ideal circle as possible.Stop the data collection.
After the full rotation, click Stop.Check calibration quality.
Confirm that the deviation indicator is acceptable. A good calibration typically shows a relatively flat Corrected Normals graph.Apply the calibration offsets.
Click Update and confirm transferring the new offset values to the instrument.Verify the calibration.
Repeat Steps 3–7 but make sure to select the Verify option before starting. The resulting circle should be centered near (0, 0) and the offset corrections should be close to zero. This confirms that hard iron magnetic influences have been compensated.Recalibrate when needed.
Always calibrate before deployment, and recalibrate if batteries are replaced, as different battery packs can have different magnetic signatures.
Without selecting Update, the calibration results will not be applied and the instrument remains uncalibrated.
Quality of calibration
The deviation/estimated magnitude error [%] gives a rough estimate of the quality of the calibration. The color of the quality indicator represent the quality:
Green: Estimated magnitude error < 2%
Corresponds to a maximum heading error of approximately 2°, consistent with the instrument specification.Yellow: Estimated magnitude error 2-6%
Corresponds to an expected heading error of approximately 2°-5°.Red: Estimated magnitude error > 6%
Indicates that the resulting heading error is likely to exceed 5°, and the calibration should be repeated.
A good calibration will also have a relatively flat Corrected Normals graph as can be seen in Figure 2 (The screenshots illustrate a compass calibration in Signature Deployment, but the same concept will apply for a calibration in Nortek Deployment.). The normals represent the normalized magnetic field vectors. Prior to calibration, magnetic offsets cause the measured field magnitude to vary during rotation, resulting in normal values different from 1. After calibration removes these offsets, the corrected magnetic vectors have constant magnitude, and the normals remain stable at 1 throughout the rotation.
The calibration seen in the example of Figure 1 is of low quality: The rotation was not carried out smoothly and with a constant velocity as can be seen by the fluctuations in the underlying blue curve in the top graph. Additionally the left calibration was carried out in the vicinity of a strong electromagnetic disturbance that did not rotate with the instrument, which deformed the ellipse. Should the quality of the calibration not reach the required accuracy, repeat the calibration or consider changing to a location with reduced magnetic influence.
When you first run the calibration routine, you will typically see that the circle will be offset from original as can be seen in Figure 3.
After the instrument offsets have been updated the calibration can be confirmed by selecting Verify and re-executing the data collection and rotation and ensuring that the circle produced is centered on (0, 0, 0) (and that the corresponding offset corrections are close to 0). This step can be used to confirm, that the hard iron influences have been compensated for.
💡 NOTE: If batteries are replaced during deployment compass re-calibration should be performed since the magnetic signature of between battery packs are different and may change as it discharges.
3D calibration
Our software applications Nortek Deployment and Signature Deployment also support 3D compass calibration. A 3D calibration will compensate for magnetic offsets affecting all three axes, whereas a 2D calibration compensates for magnetic influences in the horizontal plane only.
To perform a 3D calibration, follow the same procedure described above for the 2D calibration. However, during Step 4, the instrument must be rotated slowly around all three axes to obtain sufficient coverage of the full magnetic field.
The software visualizes the coverage in the XYZ space using blue data points. The red arrows represent individual octants of the measurement space. Each arrow turns green once sufficient data points have been collected within that octant. A minimum of 50 data points per octant is required for a complete calibration.
Demonstration of compass calibration procedure
Should the procedure be unclear, consult this demonstration created by our support team:
Offset sources
The upper graph shown in our deployment software during compass calibration shows the output of the magnetometer in counts. These raw readings can equally be seen on the right hand side under Mx, My, Mz [counts]. If there are no magnetic distortions present, the output should show a perfect circle centered on (0,0). The radius of the circle represents the magnitude of the magnetic field at the calibration location. Typically, you will see deviations from a perfect circle centered on (0,0) coming from these two sources:
Hard Iron (X/Y offset [Counts] ): This will cause a shift in the position of the ellipse away from the origin.
Hard iron or permanent magnet iron offsets are produced by materials that exhibit a constant, additive field to the earth's magnetic field, for example magnetized iron. Their offset is therefore constant and can be easily corrected for.
Soft Iron: This will deform the ellipse.
Soft iron offsets can be described as deflections or alterations in the existing magnetic field, caused for example by metals like nickel and iron or alloys like kovar and steel. These are materials that can be magnetized quickly by an external field, but lose their polarization once the external source is removed. Calibrating for soft iron effects is more complex, as it involves not just offset corrections but also scaling and sometimes even more sophisticated transformations to correct for the non-uniform distortions across different orientations.
Note that you can only correct for hard iron sources, specifically those that remain fixed and rigid to the to the magnetometer as it rotates through space. Should you for example be calibrating your instrument in a frame next to a steel vessel, you will be able to remove the offset of the frame but not the vessel as it does not follow the rotation of the instrument.
Deployment in challenging conditions
Horizontal field strength in polar regions
The magnetometer captures the strength of the horizontal component of the magnetic field. Its strength decreases progressively towards the poles, which needs to be considered when measuring in Polar regions. You can find the strength of the horizontal field in your specific location on this map or NOAA's Magnetic Field Calculator. Magnetometer measurements at a location with a field intensity below 6000nT should be used with caution. If the exact mounting position of the instrument is known, using the XYZ coordinate system is a good solution should the magnetic field intensity be to low.
Deployment next to large offset sources
When deploying the instrument next to a large offset source like a steel platform, its compass reading are not usable. Therefore it will be necessary to establish its orientation manually. Once the instrument is mounted, its true heading can be measured using some form of external source, preferably something that is not affected by the local magnetic field, such as a GPS compass. The difference between this value and what the instrument reports will be the bias (offset) introduced by the local magnetic interference and it will need to be added/subtracted from the reported estimates during post-processing. This correction can only be used, if the instrument is deployed stationary, where a constant orientation of the instrument is guaranteed.
Magnetic declination
Magnetic declination is the angle between true north (geographic north) and magnetic north (the direction a compass points). This angle varies depending on your geographic location and can cause significant discrepancies in directional measurements. The compass calibration in your instrument does not automatically account for magnetic declination at the deployment site. This means that the orientation reported by the instrument may deviate from true north, especially in areas with strong declination. Since the ENU coordinate system (East-North-Up) uses true north as a reference, magnetometer readings must be corrected for the local declination during each deployment. To identify the local declination at your deployment site use tools like NOAA's Declination Calculator.
How to correct magnetic declination
You must apply the declination correction either before or after the deployment, using one of the following methods:
1. Pre-deployment correction
Use the relevant configuration commands (refer to the Integration Manuals) to save the declination value into the instrument before deploying it.
2. Post-processing correction
When processing the recorded data, our software tools offer a setting to apply the declination correction. You can enter the local declination value during this step.
- Eastern (positive) declination: Enter a positive value.
- Western (negative) declination: Enter a negative value.
Example: For a deployment in Oslofjord, where the local declination is +4.72°, you must enter 4.72 into the processing software or save it using commands.
During post-processing, the entered declination value is automatically added to the magnetometer data, aligning the results with true north and ensuring compatibility with the ENU coordinate system. Utilize the help section in the software for detailed instructions. To access the correct data processing software for your instrument, please consult our software page.
Compass calibration in post-processing (Signature)
A compass calibration can also be done in Ocean Contour for Signature instruments in post-processing, given that the instrument has turned 360 degrees either on its way down the water, during deployment or while the instrument is retrieved. The user can select a fitting time window where a full rotation has occurred and retrieve the compass offset from the raw magnetometer data. Should your Signature be deployed on a rotating system like a buoy, no compass calibration is needed before deployment, since a higher calibration quality can be achieved using the raw magnetometer data post-deployment. Alternatively the compass calibration procedure itself can be carried out after recovering the instrument. The offsets need to be recorded and then manually added during the post-processing.
For details on the procedure please refer to the help section in the Software.
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